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What happens when lava meets sea water?

When hot lava – reaching temperatures of more than 1000C - meets water it triggers a string of chemical reactions and generates massive amounts of steam. At the same time the lava cools rapidly, forming into a type of glass.

As it shatters, the particles get picked up in the turbulent clouds of gas.

"Just like if you drop a glass on your kitchen floor, there's some large pieces and there are some very, very tiny pieces," said Janet Babb, a geologist with the Hawaiian Volcano Observatory. "These little tiny pieces are the ones that can get wafted up in that steam plume."

Scientists call the sheets of glass Limu O Pele, or Pele's seaweed, named after the Hawaiian goddess of volcano and fire.

As if that weren't enough, the high temperatures have another effect. As well as boiling away the water it heats up salt molecules left behind, such as magnesium chloride which reacts with the steam to produce hydrogen chloride gas.

How dangerous is it?

The acid is as corrosive as diluted battery acid.

Dr Alvin Bronstein from the Hawaii State Department of Health, said: "If one were to be near the laze, because of the various acids, it would be corrosive to the eyes, the nose and respiratory tract, and the skin."

Protective face masks have been distributed to protect residents from volcanic ash. Although they will protect from glass particles they will not be much use from the acid gas.

Who is at risk?

The good news is you have to be right on top of the area of laze formation to suffer the worst effects. No major injuries have yet been reported although in 2000 it contributed to two deaths, according to the Hawaiian Volcano Observatory.

"This hot, corrosive gas mixture caused two deaths immediately adjacent to the coastal entry point in 2000, when seawater washed across recent and active lava flows," it said.

Members of the media record a wall of lava entering the ocean near Pahoa, Hawaii. Photo / AP

For now, people close to the coast, on land or in boats close to shore, run the greatest risk.

The US Coast Guard is enforcing a "lava entry safety zone" keeping boats at least 300 metres from the entry of the flow into the water.